Reactor for the thermal conversion of methane

ABSTRACT

An apparatus for the thermal conversion of methane into hydrocarbons of higher molecular weight, comprising a reactor (1) of elongate form, connected on the one hand, at a first end, to an inlet for supplying gaseous mixture containing methane (process gas) and on the other, at the opposite end, to an outlet (10), the reactor comprising on the first end side a plurality of elements disposed in at least two layers disposed between two refractory walls, at least one refractory wall being disposed between the outside walls of the reactor, the layers being substantially parallel to the axis of the reactor, at least one of these layers comprising a series of sheaths (4) inside which there are electric heaters (3) which thus form a layer of heating elements, the elements being disposed in such a way as to define spaces or passages for the circulation of gaseous mixtures and/or effluents, the heaters and the sheaths being adapted to heat the said passages by successive independent cross-sections substantially at right-angles to the axis of the reactor, each cross-section comprising at least one transverse row of elements.

This is a division of application Ser. No. 07/962,453 filed Oct. 19,1192, now U.S. Pat. No. 5,365,005 issued Nov. 15, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a process for the thermal conversion of methaneinto hydrocarbons of higher molecular weight and the apparatus forcarrying out the said process. More particularly, it relates to aprocess for the thermal conversion or cracking of the methane in areactor comprising electric heating means and, by dehydrogenationthermal coupling of this molecule, permitting the production principallyof acetylene, ethylene, benzene and a little coke. Any methane sourceswell known to a man skilled in the art may be used. Natural gas maycited as a highly current source of methane. A non-exhaustive list ofthese sources is provided for instance in European Patent ApplicationEP-A-323287 in the name of the Institut Franccais du Pe/ trole, one ofthe assignees of this invention. In the majority of cases, the gascontaining methane which is introduced into the reactor contains from 1to 90% and sometimes even more of at least one other gas.

In European Patent Application EP-A-323287, there is described a processfor the thermal conversion of methane into hydrocarbons of highermolecular weight, comprising electric heating means with a transfer ofheat to the gaseous mixture containing the methane to be converted,through seal-tight walls of sheaths of ceramic material which insulatethe said heating means from the gaseous mixture containing the methane.In this process, the heating zone is heated by a supply of electricalenergy by means of electric resistors and the heat released by Joule'seffect in these resistors is transmitted mainly by radiation to thesheaths of ceramic material disposed around resistors in anon-contiguous manner. The gaseous feeds which circulate substantiallyat right-angles to the axis of the heated sheaths are heatedsubstantially by convection and by radiation. In the performance of thisprocess, two spaces are defined within the reactor:

on the one hand, the reaction space or process space outside the sheathsprotecting the resistors, in which the gaseous mixture containingmethane circulates,

on the other hand, the resister space formed by the volume comprisedbetween the actual resistors and the insulating sheaths into whichpreferably an inert gas, that is to say a gas containing no methane orany hydrocarbon capable of a thermal conversion reaction or any compoundcapable of reacting violently with methane or hydrogen, is introduced.This gas is likewise chosen in such a way that it does not damage theresistors used and does not cause accelerated ageing of the resistors.

One of the greatest problems when carrying out the thermal conversion ofmethane is linked with coke formation. Indeed, if it forms in too greata quantity, it is likely to damage the furnace before the decokingoperations can be performed and furthermore, from the economic point ofview, its formation represents a substantial loss forth with regard tothe electrical energy consumed ant the methane consumed in forming thecoke. This problem, well known to a man skilled in the art, is partlyresolved by the introduction into the gaseous mixture containing themethane to be converted of a quantity of hydrogen representing from 1 to90% by volume in relation to the total volume of gas. In spite of thisprecaution, coke formation has not been completely eradicated, mainly atthe level of the walls of the sheaths and on the other surfaces whichare at elevated temperature and which are in contact with the gaseousmixture containing the methane.

This explains why, in carrying out the methane conversion process in anelectrically heated pyrolysis furnaces, it is desirable:

to have a relatively large quantity of hydrogen present in the processzone,

to provide electrical resistors capable of delivering at hightemperature a considerable quantity of energy per unit of surface areaand per unit of time,

to have conditions conducive to satisfactory heat transfer so that thetemperature of the heating elements (that is to say the temperature ofthe surface of the sheaths in contact with the gaseous mixturecontaining methane) is not too much greater than the temperature desiredfor carrying out conversion of the methane.

In carrying out the process, it has been stipulated that it ispreferable for the resistor space to be filled by a gaseous medium suchas nitrogen, carbon dioxide or air. The use of air is only conceivableif the seal provided by the sheaths between the process space and theresistor space is perfect. Indeed, there would otherwise be asubstantial risk of forming a gaseous mixture at a very hightemperature, comprising oxygen, methane and hydrogen, which thereforeentails a risk of explosion. The provision of a completely seal tightsystem is relatively difficult and furthermore requires the use ofceramic material which offers a very high level of seal-tightness andwhich is therefore of very high quality, that is to say a ceramicmaterial the density of which is close to theoretical density and whichis free from open pores.

The use of such a ceramic material is extremely expensive, whichpenalizes the process. Therefore, one is induced to accepting the use ofsheaths of less than perfect seal-tightness and to use either nitrogenwith the not inconsiderable risk, in view of the resistor skintemperature, of the formation (in the case of silicon carbide resistors)of silicon nitride, which in principle has no effect on the mechanicalstrength of the resistors but does cause a fluctuation in theresistivity of these resistors and therefore accelerates their ageing,the more so the higher the temperature of the resistor (and therefore ofthe heating element) and the greater the amount of energy provided bythe resistor, or to use carbon dioxide gas which, even if the rate ofleakage from the resistor space into the process space is minimal, willinevitably cause problems at the stage involving separation of theproducts formed during the course of thermal conversion of the methane,complicating this stage on the one hand by their presence and on theother by the presence of carbon monoxide and water which will inevitablyform by reaction between the carbon dioxide, the methane, the coke andthe hydrogen in the process space.

SUMMARY OF THE INVENTION

One of the objects of the invention is to remedy the aforedescribeddrawbacks. The objects which it is aimed to achieve and which meet theproblems raised by the prior art are essentially the following:

to minimize coke formation, particularly on hot surfaces such as forexample the walls of sheaths enclosing the resistors,

to use as a gas in the resistor space a gas or a mixture of gasespreferably comprising a gas already present in the gaseous mixturecirculating in the process space, which makes it possible to use sheathswhich do not possess a very high level of seal-tightness,

to improve heat exchange between the gaseous mixture containing themethane and the hot surfaces in contact with this mixture,

to minimize the problems of distribution of the gases inside thereactor,

to enhance the viability of the apparatus and its ease of constructionand dismantling for decoking and maintenance of the reactor.

The present invention relates to a process and an apparatus for carryingout the process, which provide notable improvements compared with priorart constructions and processes such as for example easier, moreflexible and better controlled performance and lower costs both atinvestment level and also in respect of utilities.

More particularly, the invention relates to a process for the thermalconversion of methane into hydrocarbons of higher molecular weight in areaction zone which is elongate in one direction, comprising a heatingzone a cooling zone following the said heating zone and in which theeffluents from the heating zone are cooled, a gaseous mixture containingmethane being caused to circulate in the said heating zone in adirection of flow which is substantially parallel with the direction ofthe reaction zone, characterized in that the said heating zone comprisesbetween two walls of refractory material, at least one longitudinal zonein which the said gaseous mixture circulates, the said longitudinal zonecomprising a plurality of elements disposed in at least two layers whichare substantially parallel inter se and substantially parallel with thedirection of the reaction zone, the said elements forming in transverseprojection a cluster of triangular, square or rectangular pitch, atleast one of these layers comprising a series of sheaths inside whichthere are electric heating means, so forming a layer of heatingelements, the said heating means thus being insulated from directcontact with the gaseous mixture containing the methane and the saidheating elements being grouped in successive transverse sections eachcomprising at least one transverse row of elements, the said sectionsbeing perpendicular to the direction of the reaction zone, independentof one another and supplied with electrical energy in such a way as todefine at least two parts in the heating zone, the first part making itpossible to raise the charge to a temperature at least equal to approx.1500° C. and the second part, following on from the first part, makingit possible to maintain the charge at a temperature substantially equalto the maximum temperature to which it was raised in the first part, andaccording to which the effluent from the heating zone is cooled byintroduction into the cooling zone of a cooling fluid after which thesaid hydrocarbons of higher molecular weight are collected at the end ofthe reaction zone.

The heating zone is heated by the supply of electrical energy throughheating means such as electrical resistors; the heat given off byJoule's effect in these resistors is transmitted mainly by radiation tothe sheaths disposed around the resistances in a non-contiguous manner.These sheaths are usually of ceramic material or some other refractorymaterial which can withstand the required temperatures and the reducingand oxidizing atmospheres of the medium such as for example certain newmetal alloys from KANTHAL SA such as KANTHAL AF or KANTHAL APM, or evenof refractory cement. These sheaths may be porous or not. It is ofteneasier and less expensive to use porous sheaths which allow a gaseousmixture to pass from the resistor space to the process space. Thegaseous mixture containing methane which circulates in the heating zonein a direction which is substantially perpendicular to the axis of thesheaths is essentially heated by convection and radiation.

The thermal dehydrogenation of the methane is a highly endothermalreaction and means that a quite substantial density of heat flow must beobtained at a high level of temperature, of around 1100° to 1500° C. Itis necessary that the maximum contribution of heat be made in the zonewhere the endothermal cracking and dehydrogenation reactions areperformed; furthermore, having regard to the reactivity of the productsformed, such as acetylene or ethylene, it is necessary to have arelatively short controlled contact time followed by a rapid quenchingin order to obtain a temperature profile of the "square" type and inorder to avoid excessive coking.

Heat exchanges are one of the key elements for this type of highlyendothermal reaction where it is necessary to transfer quite substantialamounts of energy from the resistors to the gaseous mixture containingmethane and hereinafter referred to as the process gas. During apreliminary study carried out of heat exchanges in a pyrolysis furnaceconstructed according to the model used in the present invention, it wasnoted that the exchange of heat from the resistance to the sheath is anessentially radiative exchange but on the other hand there is virtuallyno radiative exchange between the sheath and the process gas. Indeed,this latter is normally essentially constituted by a hydrogen-methanemixture, a mixture which absorbs virtually no or very little radiationemitted by the sheaths. The transfer of heat between the process gas andthe sheaths is therefore in the case in question in the presentinvention essentially a transfer by convection. In such a case, thequality of the heat exchanges will be linked directly to the surfacearea available for the exchange and to the surface area/volume ratio.

Thus, if the surface area for exchange is relatively small, it will benecessary, in order to obtain a given process gas temperaturecorresponding to a previously chosen conversion level, to increase thetemperature of the sheaths in proportions which will be greater as thesurface area becomes smaller, which implies an increased risk of cokeformation and also the need to raise the temperature of the resistors,which causes a more rapid ageing of these resistors or even if thepreviously chosen rate of conversion is very high, the quantity ofenergy to be transferred becomes very great and the risk of theresistors deteriorating is quite considerably aggravated.

The walls play an important part in heat exchange since they are capableof absorbing the rays emitted by the sheaths; consequently thetemperatures of sheaths and of the walls have a tendency to balance. Itis then possible to increase considerably the surface area available forexchange by modifying the design of the device as follows: whereas inthe initial conception, the sheaths protecting the resistors and makingit possible for the transfer of heat to the process gas were preferablyarranged in quincunx, according to the present invention they will moreoften than not be aligned which makes it possible to make up n rows orlayers of m resistors in the longitudinal direction (for a total numberof resistors equal to n×m) and in a preferred embodiment it will bepossible to associate a certain number of preferably aligned pseudoheating elements which makes it possible to constitute z rows or layersof y pseudo heating elements (for a total number of pseudo heatingelements equal to z×y), which means that at least one longitudinal zoneand more often than not at least two longitudinal zones will be formed,each comprising at least two layers of elements of which at least onecomprises heating elements, each zone being separated from the next by awall of refractory material. By radiation, the temperature of thesewalls increases and has a tendency to attain the same level as that ofthe sheaths enclosing the resistors. These walls will therefore likewiseparticipate in the heating of the process gas by convection and the samewill apply to the pseudo heating elements if these are provided. Thus,in this embodiment, as the heat exchange area is substantiallyincreased, it will be possible to obtain the same process gastemperature with a relatively lower sheath and wall temperature, whichconsequently makes for a reduction in coke formation. In the presentdescription, the term heating element designates the assemblyconstituted by a protective sheath and at least one resistor inside thesaid sheath, and the term pseudo heating element denotes an element of arefractory material usually of the same height as the heating elementand of which the cross-section more often than not has the same form andthe same surface area as those of heating elements or a similar form ora derived form and a different surface area. For example, if the heatingelements have a circular cross-section, the pseudo heating elements mayhave a cross-section which is circular, semi-circular or whichcorresponds to a quarter of a circle.

In this embodiment, convective exchanges between the process gas and thewalls are generously enhanced and may be still further improved byimparting considerable velocities to the gas and by creating zones ofturbulence. Increasing the velocity of the gases may be achieved forexample by using walls the shape of which is favourable to this increasein velocity and the formation of turbulent zones. Walls of particularforms are shown in FIG. 1A, by way of non-restrictive examples. Thewalls and the pseudo heating elements are normally of a refractorymaterial. Any refractory material may be used for producing the wallsand these pseudo elements and by way of non-limitative example,zirconium, silicon carbide, mullite and various refractory cements maybe cited. In a preferred embodiment, the heating elements will comprisea sheath of a ceramic material.

In view of the fact that it is by no means indispensable to have a tightseal at the level of the walls, since the composition of the gas isvirtually identical on each side of the walls, this embodiment requiresa minimal increase in the cost of the furnace. Indeed, on the one handit is not necessary to have especially thick walls nor for theirconstruction to be particularly complex, but on the other hand theoverall dimensions of the furnace do increase very much becauseessentially the width of the furnace is due to the width of the sheaths.By way of example, the sheaths may have a width of around 150 mm for awall thickness having a width of around 50 mm, which only means anincrease in the width of the furnace of less than about 30%. Moreover,it is preferably for each wall to comprise at least one means ofpermitting the pressures situated in the longitudinal zones on eitherside of the wall to be balanced. By way of example of a simple buteffective means which makes it possible to balance the pressures, it ispossible to quote the creation of zones comprising one or a plurality ofperforations or porous zones.

An additional advantage of this embodiment comprising walls is that ofallowing simpler construction of the furnace, the vertical walls notonly improving the transfer of heat by convection but making it possibleto support the roof of the furnace. It should be noted that according tothe preferred embodiment of the invention, the pseudo heating elementslikewise improve the transfer of heat and likewise participate insupporting the furnace roof.

According to one of the characteristic features of the invention, theelectrical resistors which supply heat to the heating zone areindependently supplied with electrical energy either individually or intransverse rows or even in small groups, in order to define heatingsections along the heating zone and in order thus to be able to modulatethe quantity of energy provided throughout the length of this zone. Theheating zone is normally composed of 2 to 20 heating sections andpreferably 5 to 12 sections. In the first part of this zone, the gaseousmixture containing methane, previously heated to approx. 750° C., isnormally raised to a temperature which is at most equal to approx. 1500°C. and advantageously between 1000° and 1300° C. (the start of theheating zone is situated at the place where the charge is introduced).

These heating sections are modulated in a conventional manner; theheating elements corresponding to the aforesaid sections are generallysupplied by thyristor modulator assemblies. Transformers make itpossible if necessary to adapt the voltages a priori while themodulators permit of fine and continuous adjustment of the injectedpower.

In order to allow the assembly to be regulated, each heating section maybe provided with an insertion pyrometer with a thermocouple suitable forthe temperature level involved; these pyrometers are disposed in thespaces in which the charge circulates the data are transmitted to thecontroller which operates the thyristor modulator.

In the first part of the heating zone, the electrical energy servesalmost exclusively to raise the reaction mixture from its initialtemperature (for example approx. 750° C.) to the temperature at whichthe endothermal dehydrogenating coupling reactions of the methane occur(for example approx. 1200° C.). It is therefore at the commencement ofthe second part of the heating zone that it is necessary to providemaximum energy to the reaction environment, which is easily achieved forinstance by modulation of one or more heating sections and/or by usingmodules (described hereinafter) for manufacturing the furnace,comprising a different number of pseudo elements according to whetherthey are situated at the start of the first part of the heating zone ortowards the middle or the end of this zone and similarly according totheir position in the second part of the heating zone.

The length of the first part of the heating zone normally representsfrom 20 to 80% of the total length of the heating zone, advantageouslyfrom 30 to 70%. The electrical energy supplied to this first of theheating zone is such that it generates a considerable temperaturegradient, normally from approx. 0.5 to approx. 25° C./cm andadvantageously approx. 1 to approx. 20° C./cm.

In the second part of the heating zone, the electrical energy providedto the various heating sections of this zone is modulated in such a waythat the fluctuation in temperature along this zone is minor, usuallyless than approx. 50° C. (+or -25° C. around the desired value) andadvantageously less than approx. 20° C. Furthermore, the use ofdifferent transverse heating sections (for example as well as the use ofdifferent modules comprising a greater or lesser number of pseudoheating elements) which are independent of one another makes itpossible, at the level of the second part of the heating zone, to applymaximum heat energy at the place where the major part of the endothermalreactions takes place while maintaining a virtually uniform temperaturein the rest of the heating zone.

The length of the heating zone is usually approx. 50 to approx. 90% ofthe total length of the reaction zone.

Particularly under the above-described heating conditions, a quitesubstantial heat flow is achieved at an elevated temperature level. Thisnormally entails a particular choice for the material used for theresistors which, in addition to being resistant to the atmosphere inwhich the resistors are immersed under operating temperature conditions,have to be capable of delivering a relatively substantial output perunit of surface area. As an example of a material which can be used formaking resistors, it is possible to quote silicon carbide, boronnitride, silicon nitride and molybdenum bisilicide (MoSi₂). Usually, itis preferable to use molybdenum bisilicide resistors which offernumerous advantages when they are used at high temperature:

they accept a considerable charge (power emitted per unit of surfacearea) which may be as much as 20 W/cm²,

they can work at a very high temperature

they display negligible ageing in course of time,

they readily withstand reducing atmospheres at elevated temperatures. Inthe process according to the invention, the heating zone is followed bya cooling (or quenching) zone in order very rapidly to reduce thetemperature of the effluent from the zone to approx. 300° C. forexample.

According to an embodiment, direct quenching is carried out; thereaction effluent leaves the heating zone and is very rapidly cooled bybeing brought directly into contact with a cooling medium which isinjected into the effluent by means of at least one injector, normallyof a ceramic material, disposed at the periphery of the reactor. As acooling medium, it is possible to use gases of liquefied petroleum,propane, hydrocarbon oils or water. Propane is the preferred quenchinggas because it can also be partially cracked and so contribute to theformation of products such as ethylene. The total effluent resultingfrom the mixture is then collected and separated.

In accordance with a preferred embodiment, the reaction effluentemanating from the heating zone is cooled by being brought into indirectcontact with a cooling medium, for example by causing the said medium tocirculate through seal-tight ducts inside the cooling zone. All thesecharacteristic features, by virtue of this process, make it possible toachieve a thermal conversion of the methane into acetylene, ethylene andbenzene products which is achieved with a good rate of conversion andconsiderable selectivity of these products.

The hydrocarbon feeds which can be used within the framework of theinvention are gaseous charges under normal temperature and pressureconditions, usually comprising a percentage by volume of methane of atleast 10%, for example from 10 to 99%, and more often than not from 20to 99% and preferably from 30 to 80%. As stipulated hereinabove, therest of the batch in almost all cases comprises a proportion of hydrogenby volume which ranges most often from 1 to 90%. It may likewisecomprise other gases such as for instance aliphatic hydrocarbons,saturated or otherwise, comprising a number of atoms equal to or greaterthan 2 such as ethylene, ethane, propane or propylene. It may alsocontain nitrogen, carbon dioxide or carbon monoxide.

While remaining within the framework of the invention, it is possible toadd dilution steam to charges defined hereinabove; the ratio by weightof dilution steam to hydrocarbon charge is generally around 0.1:1 to1:1.

The charges to be treated have a dwell time in the reaction zone whichis normally approx. 2 milliseconds to approx. 1 second and preferably ofapprox. 30 to approx. 400 milliseconds. As a sealing gas, that is to sayas a gas which is introduced into the resistor space (this space is thatwhich was defined hereinabove in connection with the analysis of theEuropean Patent Application EP-A-323287), and preference will be givento a gas which will make it possible to obtain the longest effectivelife of the resistors while entailing, due to the leakage of the saidgas into the process space, a minimum of complications concerning thereaction itself (minimum coke formation) and also the separation of theproducts downstream of the reactor, sometimes referred to as a pyrolysisfurnace. For example, it is possible to choose as a sealing gasnitrogen, CO₂, hydrogen or a mixture of 2 or more of these gases. It islikewise possible to use as a sealing gas one of the gases mentionedhereinabove containing water vapour. Usually it is preferred tointroduce into the sheaths enclosing the resistors a gas containinghydrogen. This gas may be substantially pure hydrogen, industrialhydrogen or a mixture of hydrogen with another inert gas such as forexample nitrogen, helium, argon, steam or CO₂ gas. Preferably, pure orindustrial hydrogen or a mixture of helium and hydrogen or a mixture ofargon and hydrogen or a mixture of water vapor and hydrogen is usedwhich normally contains at least 5% and preferably at least 10% byvolume of hydrogen. When a mixture of nitrogen and hydrogen is used,this usually contains at least 25% and preferably at least 50% by volumeof hydrogen.

In a preferred embodiment, the electric heating means will be insulatedfrom direct contact with the gaseous mixture containing methane bysheaths into which a gas containing hydrogen is introduced, the saidsheaths being of suitable permeability and the gas being introduced intothe interior of the sheaths being at such a pressure that there is atleast at certain points a diffusion of at least a part of the hydrogencontained in the gas introduced into the resistor space, towards theprocess space, that is to say from the interior of the said sheaths tothe outside of the said sheaths, this hydrogen possibly being dilutedthen in the gaseous mixture containing the methane.

It would not be a departure from the scope of the invention if thepermeability of the sheaths were such that it allowed the diffusion ofall the gaseous compounds contained in the gas introduced into theresistor space towards the process space. This permeability may resultfrom a seal on each sheath, provided voluntarily in an incomplete wayand/or the use of a material constituting the sheaths which has an openporosity allowing the hydrogen to pass through, that is to say in otherwords a material which is permeable to hydrogen. More often than not, itis recommended to use a permeable material.

Thus, in accordance with a preferred embodiment of the invention, thesheaths insulating the electric heating means from direct contact withthe gaseous mixture containing the methane are made from a porousmaterial of sufficient porosity to allow the hydrogen to diffuse throughthe said sheaths. Thus, these sheaths are preferably made from a porousceramic material having an open porosity of at least approx. 1% and ofat most approx. 40% by volume in proportion to the volume of the wall ofthe sheath and usually of approx. 5% to approx. 30%.

The use of substantially pure hydrogen which diffuses at least partlytowards the process space has a number of advantages. It does notcomplicate the separations downstream of the pyrolysis furnace becausethe gas to be cracked is normally a mixture of methane or natural gasand hydrogen in a proportion by volume of preferably 10 to 80% hydrogenand most frequently 30 to 70%.

The introduction of hydrogen along the pyrolysis furnace makes itpossible to reduce the overall size of the furnace. Indeed, if a certainproportion of hydrogen in the cracked gas is intended, then at theintake to the furnace the proportion will be reduced and for one and thesame dwell time to be respected, the reaction volume will be less,therefore the size of the furnace will also be reduced. But, moreover,this embodiment will result in an increasing proportion of hydrogenalong the pyrolysis furnace, which represents an advantage from thepoint of view of the kinetics of cracking and the stability of theproducts because at the beginning of the furnace, too great a quantityof hydrogen would excessively inhibit the cracking reactions, but at theend of the furnace, when there is a substantial quantity of formedproducts, particularly ethylene and acetylene, it is advantageous tohave a greater quantity of hydrogen in order to avoid coke formation.The desired effect will indeed be achieved if there is hydrogenpenetrating the process zone at the level of each seal (at least one persheath) on each sheath protecting the resistors and/or through the wallof the sheaths by diffusion.

Furthermore, in accordance with a preferred embodiment of the process ofthe present invention, in view of the fact that it is not indispensableto seek the most perfect seals possibly between the process space andthe resistor space, the cost of production of the furnace is reduced bylikewise reducing the thermo-mechanical stresses at the level of thesheath flanges, which enhances the viability of the apparatus as awhole. Another advantage resides in choice which it is then possible tomake with regard to the sheaths protecting the resistors, dividing theprocess space from the resistor space. Indeed, when nitrogen or CO₂ isused as the sealing gas, it is necessary for many reasons to limit theconsumption of this gas, that is to say the leakage of the said gas fromthe resistor space into the process space. This is normally achieved byseeking as perfect a seal-tightness as possible, particularly at thepoint where the sheaths are joined to the rest of the furnace. This isalso achieved by using ceramic sheaths, particularly sheaths of siliconcarbide which are as seal-tight as possible, that is to say of very goodquality and therefore very expensive.

It is indeed well known to a man skilled in the art that there are manyvarieties of ceramic material and in particular silicon carbide, whichoriginate from vastly differing qualities of constituent powder andsintering conditions. Without wishing to go into detail, however, it maybe noted that one of the quality criteria is linked to the leastpossible porosity remaining after sintering. It is well known that if apart of this porosity is found to be closed, that is to say has noeffect on the overall seal-tightness of the material, another and notinconsiderable part, above all for the most ordinary silicon carbide, isan open porosity and that in particular there is at high temperature adiffusion of the hydrogen through this material. It will therefore bereadily understood that when a gas such as nitrogen or CO₂ is used asthe sealing gas, a very high quality silicon carbide has to be usedwhich has a density close to theoretical density, in other words freefrom any open pores, in order to avoid firstly a leakage of the said gasfrom the resistor zone into the process zone and secondly as the partialpressure differential of the hydrogen is positive in theprocess-resistor sense, a diffusion of the hydrogen contained in theprocess gas towards the resistor space.

The use of sheaths made from ceramic material, particularly siliconcarbide of average quality, comprising open pores amounting to at leastapprox. 1% by volume (for example approx. 20% by volume) is thus notonly possible but even desirable, which reduces the cost of producingthe furnace. Furthermore, the very existence of this open porositycreates on the surface of the ceramic sheath, on the process space side,a partial hydrogen pressure which to a certain extent insulates thesurface of the ceramic from the process gas which, without wishing to bebound by any theory, explains the substantial reduction in cokeformation since this latter normally forms mainly on the surface of thesheaths, the products formed being in contrast in a local atmospherewhich is richer in hydrogen and therefore less favourable to cokeformation.

The term `open porosity` in the description of the present inventionrelates to the porosity constituted by microcavities included in thesolid ceramic parts in question, the adjective `open` signifying thatthere is freedom of passage on the one hand between the majority of thesaid microcavities and on the other between the said microcavities andthe inner and outer surfaces of the parts in question; the idea of freepassage must also be considered as a function of the nature of theenvironment and the physical conditions in which the ceramic is present.In particular, for small size molecules such as hydrogen or helium, freepassage will be easy, the more so since if there is a difference inpressure between the two surfaces of the ceramic part. In this case, thepart is said to be permeable, to hydrogen for example, or is notfluid-tight. In the description of the present invention, the term`closed porosity` refers to the porosity consisting of microcavitieswhich do not communicate with the surface of the part. In that case,this closed porosity only results in an overall diminution of thedensity of the part.

Another object of the invention is the apparatus for carrying out theprocess. This apparatus may likewise be used for carrying out otherendothermal reactions which normally occur at temperatures above approx.600° C. and for example approx. 700° to approx. 1450° C., with dwelltimes amounting to 2 milliseconds up to a few seconds, for example up to20 seconds.

More particularly, the invention relates to an apparatus for carryingout the process and which comprises a reactor (1) of elongate formaccording to an axis of preferably square or rectangular cross-section,comprising at a first end means of supplying a gaseous mixture and atthe opposite end means of discharging the effluent produced and, betweenthese two ends, means of supplying a cooling medium, characterised inthat the said reactor comprises in a first part (the same side as thefirst end) at least one longitudinal zone between two walls ofrefractory material, which are substantially parallel with each otherand substantially parallel with the axis of the reactor, the saidlongitudinal zone comprising a plurality of elements, disposed in atleast two layers, substantially parallel inter se and substantiallyparallel to the axis of the reactor, at least one of these layerscomprising a series of sheaths (4) inside which there are electricheating means (3) which thus form a layer of heating elements, the saidelements being disposed in such a way as to define between them and/orbetween the layers which they form and/or between them and the walls ofthe spaces or passages for circulation of the gaseous mixtures and/oreffluents, the said heating means and the said sheaths being adapted toheat the said passages in successive independent transverse sectionswhich are substantially at right-angles to the axis of the reactor, eachtransverse section comprising at least one transverse row of elementswhile the said reactor comprises in addition means for the automaticcontrol and modulation of the heating and which are connected to thesaid heating means, and comprising in a second part (8) (the side of theopposite end) contiguous to the first part, means (9) of cooling theeffluent and connected to the said cooling medium supply means.

According to a preferred embodiment, the said apparatus will comprisemeans of introducing, at a suitable pressure, a gas containing hydrogeninto the interior of the sheaths (4) and the said sheaths will besheaths of sufficient permeability that, at least at certain points, atleast a part of the hydrogen is allowed to diffuse from the interior ofthe said sheaths to the exterior of the said sheaths, this hydrogen thenbecoming diluted in the said gaseous mixture.

The means of introducing the gas at a suitable are those known to a manskilled in the art. They may moreover comprise means of regulating andmonitoring the pressures prevailing inside and outside the said sheaths.The said cooling means are means adapted to cool the effluent leavingthe first zone, by direct or by indirect contact.

The sheaths enclosing to resistors, normally in a non-contiguousfashion, may be disposed in a superposed arrangement or in quincunx andmay in a transverse projection form a cluster with a triangular, squareor rectangular configuration.

The total number of layers comprising heating means and the number ofheating means in each sheath and per layer are not decisive in theprocess; obviously, they are a function of the dimensions of the heatingmeans, the sheaths which enclose them and the walls separating thelongitudinal zones. The heating elements may be identical to one anotheror different, both in their dimensions and in their heating output. Byway of example, a heating element may comprise from 1 to 5 and morefrequently from 1 to 3 resistors inside the sheath.

The number of heating elements determines the maximum electric poweravailable for a given reaction volume, and similarly has its effect onthe dwell time of the charge; it will be chosen as as function of therate of flow of charge admissible, taking these parameters into account.

Within the framework of the present invention, it is possible toconstruct the whole of the reactor, heating zone and quenching zone,either in monobloc form, or even by the juxtaposition--contiguous orotherwise--of various modules, more often than not of identical form,which are assembled to one another by any usable means, for example withthe aid of flanges. For example, the reactor may comprise at least twolongitudinal zones formed by the juxtaposition in the longitudinal senseand in the transverse sense of a series of modules each comprising atleast one longitudinal wall of refractory material. In a preferredembodiment, each wall of refractory material separating two adjacentlongitudinal zones comprises at least one means permitting the balancingof pressures between the two zones. When the reactor is formed by theassociation of modules, these latter will preferably be assembled in anon-sealing-tight fashion, so that gases are able to pass from onelongitudinal zone to the longitudinal zone situated on the other side ofthe wall, at the point of assembly. The reactor normally comprises from1 to 20 and preferably from 2 to 8 longitudinal zones. One of theadvantages of producing the furnace by the association of consecutivemodules resides in the unity which is derived therefrom. For example, itis possible to associate with each module one furnace roof element andalso one heating and power control unit, the module then constituting aheating section. Furthermore, dismantling and maintenance of the furnaceare simplified as a result. According to another embodiment, a pluralityof modules may be associated in order to form a heating section. It islikewise possible to install the various modules in the furnacecomprising walls, continuous or otherwise, which have been previouslypositioned. The electric heating means which can be used within theframework of the present invention are preferably heating resistors theconstituent material of which must be resistant to the atmosphere inwhich they are immersed. In a preferred embodiment, resistors are usedwhich are produced from a material which will withstand an overallreducing atmosphere, up to temperatures of the order of 1500° C.; it ispreferable to use resistors of molybdenum bisilicide, for examplehair-pin shaped resistors. The elements present in each longitudinalzone are preferably substantially cylindrical or tubular elements, allof which have substantially the same outside diameter and substantiallythe same height, and in which those enclosing the heating means andforming a layer of heating elements are cylindrical or tubular sheathshaving an inside diameter D of approx. 1.2 to approx. 8 times and moreoften than not approx. 1.5 to approx. 4 times the maximum diameter d ofthe circle embracing the said heating means and the other elements arehollow elements such as cylindrical or tubular sheaths or solidcylindrical elements.

These sheaths of refractory material are most frequently of a ceramicmaterial. It is possible to use ceramics such as mullite, cordierite,silicon nitride, silicon carbide, silica or alumina; silicon carbide isthe preferred material because it offers good heat conductivity. Thelongitudinal zones are separated by walls manufactured from a materialwhich may be the same as that used for producing the sheaths but whichis often different, particularly on grounds of furnace production cost.In a preferred embodiment of the reactor, each longitudinal zone isformed by the juxtaposition of a series of constituent modules of squareor rectangular cross-section, each comprising at least two elementswhich form a transverse row, of which at least one is formed by a sheath(4) inside which there is an electric heating means (3), and constitutesa heating element (19), the said elements being disposed in such a waythat they define between them and/or between them and the walls of thesaid zone, spaces or passages for the circulation of gaseous mixturesand/or effluents, and the said modules being juxtaposed in such a waythat the elements form between two walls of refractory material,substantially parallel inter se and substantially parallel with the axisof the reactor, at least two layers which are substantially parallelinter se and substantially parallel with the axis of the reactor.

Each constituent module may comprise at least two transverse rows of twoor three heating elements which are so disposed that the juxtapositionof these modules makes it possible to obtain at least two layers ofheating elements, the said layers being perpendicular to the axis of thereactor and the said elements forming in transverse projection a clusterof triangular, square or rectangular configuration.

According to another embodiment, each constituent module may comprise atleast two transverse rows of which at least one is formed by heatingelements and of which at least one other, contiguous upon a row ofheating elements, is formed by pseudo heating elements of a refractorymaterial, the said elements being disposed in such a way that thejuxtaposition of these modules makes it possible to obtain at least twolayers of elements, the said layers being perpendicular to the axis ofthe reactor and the said elements forming in transverse projection acluster of triangular, square or rectangular configuration. According tothis last-mentioned embodiment, each constituent module may comprise onits periphery pseudo heating elements, the said pseudo heating elementshaving a cross-section such that by juxtaposition of the modules, thosesituated at the level of the edge at which the said modules arejuxtaposed form pseudo elements of substantially the same cross-sectionas the heating elements, those contiguous upon one side having across-section which is less than the cross-section of the heatingelements and preferably a cross-section equal to approx. half thecross-section of the said heating elements and equal to approx.one-quarter in the case of those which are contiguous upon two sides ofthe module.

The elements are disposed in parallel layers which are substantiallyperpendicular to the direction of flow of the process gas, preferablysubstantially aligned, so that the distance separating two adjacentelements is as small as possible while taking into account the vitalfactors of admissible loss of charge; the distance between the elementsof two adjacent layers or the distance between the elements in one layerand the nearest wall is normally the same as that between twoconsecutive elements in a given layer.

This distance will normally be such that the passages formed between theelements or between the nearest wall, passages in which the gaseousmixture containing methane circulates, will measure approx. 1 to approx.100 mm and most frequently approx. 5 to approx. 40 mm.

According to a particular embodiment of the invention, the free spacesor passages defined hereinabove intended for circulation of process gas,are at least partially occupied by linings, usually of ceramic material,which are preferably heat conductive. Thus, for a given type of reactor,it is possible to reduce the dwell time of the charge in this reactorwhile homogenising the flow of gaseous mixture and providing a betterdistribution of the dissipated heat. These linings may take variousforms and may for example take the form of rings (Raschig rings, Lessingrings or Pall rings), saddles (Berl saddles), bars, closed cylindricaltubes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the description ofsome embodiments, given purely by way of illustration and implying nolimitation, which will be given hereinafter and in which reference ismade to the appended drawings in which similar parts are designated bythe same reference letters and numerals. In the drawings:

FIG. 1A shows a longitudinal section through a reactor, on a plane atright-angles to the axis of the elements. In the case of FIG. 1A, thisreactor only comprises heating elements in the heating zone;

FIGS. 1B and 1C show a longitudinal section through a reactor taken onthe axis of the elements;

FIG. 2 illustrates a detail of the heating zone in a plane identical tothat in FIGS. 1B and 1C;

FIGS. 3A, 3B, 3C and 4A, 4B and 4C show a longitudinal section throughvarious modules of the furnace construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with one embodiment, FIG. 1A shows a substantiallyhorizontal reactor (1) of elongate form and of rectangularcross-section, comprising a distributor (2) which makes it possible,through an inlet aperture (5), to supply the reactor with the gaseousmixture to be reacted. This gaseous mixture which contains for example50% methane, has been preheated, preferably by convection, in aconventional preheating zone, not shown in the drawing. The reactorcomprises two longitudinal zones (20) separated by a wall (22), ideallyof a ceramic material, each wall having a plurality of heating elements(19), comprising electric heating elements (3) enclosed in sheaths (4)disposed in parallel layers and forming in one plane (the plane of thedrawing) a cluster having a square configuration. These layers definetransverse heating sections which are substantially perpendicular to theaxis of the reactor defined according to direction of flow of thecharge. Likewise, on its sides parallel with the direction of flow ofthe charge, this reactor has walls of a form which is adapted to createturbulence, comprising cells at the level of each element (19). Theheating sections are supplied with electrical energy independently,thanks to a pair of electrodes (6a, 6b in FIG. 1C), pyrometricthermocouple probes (7 in FIGS. 1B and 1C) are housed in the spaces inwhich the charge circulates between the elements (19) and they make itpossible automatically to regulate the temperature of each heatingsection by a conventional regulating and modulating device not shown inthe drawings.

In the first part of the heating zone, the elements are heated in such away that the temperature of the charge changes rapidly from 750° C.(preheating temperature) to approx. 1200° C.; this progressive heatingzone generally represents about 65% of the total length of the heatingzone; then, the gaseous mixture circulates in the second part of theheating zone in which the temperature is generally maintained at aconstant level which is substantially equal to that attained at the endof the first heating zone, in other words generally 1200° C. approx. Tothis end, the electric power supplied to several heating sections whichconstitute the second part of the heating zone is modulated; thus it ispossible to achieve a variation in temperature which does not exceedapprox. 10° C. around the desired value. The length of this secondheating zone represents approx. 35% of the total length of the heatingzone.

At the outlet of the heating zone, the reaction effluent is cooled in acooling zone (8). It comes in contact with a quenching agent such aspropane, introduced via quenching injectors (9) disposed on theperiphery of the reactor (1) and connected to an outside source ofpropane, not shown. All the effluent gas is cooled to a temperature ofapproximately 500° C. and collected by an outlet orifice (10) at the endof the reaction zone (1).

According to another embodiment, not shown, the effluent may be cooledby circulating through seal-tight ducts disposed in the zone (8) throughwhich the quenching agent flows, these ducts being connected to theexternal source of quenching agent.

FIG. 1B shows, for a horizontal reactor, the same elements as thosedescribed in connection with FIG. 1; in addition, a protective casing(11) is shown which comprises an aperture (12) through which isintroduced the gas containing hydrogen and an aperture (13) providedwith a valve (24) which makes it possible to regulate the flow of gascontaining hydrogen. This casing (11) is fixed on the metal frame of thereactor (1) and encloses the heating elements, formed by the assembly ofelectrical resistors and sheaths containing them, except for the ends ofthe electrical resistors through which the electrical energy issupplied. The resistors (3), of hairpin shape, are positioned in thesheaths (4) by means of washers (18), of ceramic fibre for example,comprising passages (23) allowing the gas containing the hydrogen toenter the space comprised between the resistors and the sheaths.

In accordance with the recommended embodiment, the reactor will comprisea casing (11) partitioned in such a way that transverse zones aredefined each of which is separately supplied with a sealing gas. Thisembodiment makes it possible to limit the effect of the loss of chargeof the reactor on the rate of leakage of sealing gas from the resistorspace into process space and therefore permits of better control of thisrate of leakage. For a vertical reactor, FIG. 1C shows the same elementsas those described in connection with FIG. 1A; in addition, the drawingshows the protective casings (11) fitted with apertures (12) and (13)allowing circulation in the casings of the gas containing hydrogen whichpenetrates the resistor space through the apertures (23) in the washers(18) which assist with positioning of the resistors. The apertures (13)are fitted with valves (24) which allow easier control of the flow ofhydrogen-bearing gas. The circulation of the gas containing the hydrogenis normally carried out in a slight over-pressure in relation to thepressure of the process gas inside the reactor, so ensuring a perfectlycontrolled atmosphere and a better diffusion into the process space ofthe hydrogen contained in the gas.

The pressure could be virtually equal to that of the process gas and inthis case as in the case of an overall over-pressure, it is usuallypreferable for the partial pressure of the hydrogen to be slightlyhigher in the resistor space than in the process space in order to becertain that the hydrogen diffuses properly from the resistor space intothe process space. The difference in the partial pressures of hydrogenwill more often than not be such that the partial pressure of thehydrogen within the gas contained in the resistor space is at least 0.1%and preferably at least 1% greater than that of the hydrogen containedin the process gas. The difference in absolute pressure between theresistor space and the process space, or over-pressure, will preferablybe such that the pressure in the resistor space is at least 0.1% andmore often than not at least 1% greater than the pressure in the processspace. It is not necessary to have a very high over-pressure and in mostcases the pressure in the resistor space remains less than twice thepressure in the process space.

FIG. 2 shows a detail of an embodiment of the heating zone according tothe invention. As an electric heating means, resistors (3) ofcylindrical shape are used. At each of their ends, these resistors havecold zones and a part of the central zone which is the hot zone forinstance represents approx. 68% of the total length.

A reactor of rectangular cross-section is produced, its walls beingconstituted by insulating refractory cement (14) and a metallicframework (15). A circular hole is made in the two opposite lateralwalls and through the holes is passed a sheath (4) of ceramic materialfor example, the diameter of which is twice that of the electricalresistor (3). The sheath (4) is positioned by means of a compressiongland system (16) acting in a groove at the level of the metallicframework on a cord of refractory material (17), for a cord of ceramicmaterial. The resistor (3) is positioned in the sheath (4) by means ofwashers (18), of ceramic fiber, for example, comprising orifices (23) toallow passage of the gas containing hydrogen and introduced into thecasing (11) via the duct (12) and into the resistor space (34).

The hot zone of the resistor (3) is so positioned that it does notpenetrate the throughway passing through the insulating concrete wall.It is not vital to use a cord (17) at the compression gland level sincewithin the framework of the invention the purpose of this is to act as apositioning means, its main object being to ensure the most perfectsealing-tightness possible between the inside and the outside of thereactor. Furthermore, this compression gland may be advantageouslyreplaced by a simpler means of positioning sheaths such as for examplesimple washers of refractory material.

Thus, there are a certain number of heating resistors which are sheathedin walls, for example in ceramic material, in successive horizontalrows, these rows preferably being so aligned that, on the lateral wallsof the furnace, they form a cluster of square or rectangularconfiguration. A casing (11) from which only the ends of the resistorsand/or their electricity supply (6) projects, is traversed by a flow ofgas containing hydrogen. FIGS. 3A, 3B, 3C, 4A, 4B and 4Cdiagrammatically show a longitudinal section through six types ofmodules which can be used for constructing the pyrolysis furnaceaccording to the invention, the drawings showing a plane at right-anglesto the axis of the elements. In the case of FIGS. 3A, 3B, 3C, thesemodules only comprise heating elements (19). In the case of FIGS. 4A, 4Bad 4C, these modules comprise heating elements (19) and pseudo heatingelements (21). Each module will more often than not comprise from 2 to30 and preferably 5 to 15 of these elements. Each module will normallycomprise from 1 to 30 and most often from 5 to 11 heating elements.

The module shown diagrammatically in FIG. 3A has three transverse rowsof heating elements. By association of modules of this type in adirection at right-angles to the transverse rows (that is to sayparallel with the direction of flow of the gases), it will be possibleto form a longitudinal zone comprising three layers of heating elements.It is similarly possible to associate modules of this type in accordancewith two perpendicular directions and form a longitudinal zonecomprising for example six layers of heating elements if the moduleshave been associated two-by-two in the direction at right-angles to thethree transverse rows. The module shown diagrammatically in this FIG. 3Aconsists of elementary units each comprising one transverse row of threeheating elements. In horizontal projection, the constituent elements ofthis module form a cluster of square configuration.

The module shown diagrammatically in FIG. 3B comprises five transverserows of heating elements. This module is formed by transverse rows AA'comprising three heating elements separated from one another bytransverse rows BB' of two heating elements. The constituent elements ofthis module, in horizontal projection, form a cluster of triangularconfiguration. The module shown diagrammatically in this FIG. 3Bconsists of elementary units each comprising two transverse rows, one ofthree heating elements and the next one of two heating elements. Thismodule further comprises a refractory wall (22).

The module shown diagrammatically in FIG. 3C has five transverse rows ofheating elements. This module only differs from that showndiagrammatically in FIG. 3B by the fact that the oblique distancebetween these elements is equal to the distance between these elementsin the transverse direction and the distance in the direction ofcirculation of the gases between the elements is greater than thedistance in the transverse direction.

The use of modules such as those shown diagrammatically in FIG. 3B andthose shown diagrammatically in FIG. 3C having a geometry in quincunxwith elements situated at a distance p from one another in the directionof flow of the gases and transversely and at an oblique distance(according to an axis substantially at 45° to the axis AA' of thetransverse rows), p/2 in the case of the module shown diagrammaticallyin FIG. 3B and p in the case of the shown diagrammatically in FIG. 3C,makes it possible to create zones of constant velocity for the gases(module according to FIG. 3B) and zones of variable velocity for thegases (module according to FIG. 3C).

The module shown diagrammatically in FIG. 4A comprises three transverserows AA' of heating elements and three transverse rows CC' of pseudoheating elements. By association of modules of this type according to adirection at right-angles to the transverse rows (that is to sayparallel with the direction of gas flow), it is possible to form alongitudinal zone comprising layers FF' of heating elements and layersEE' of pseudo heating elements. The module shown diagrammatically inthis FIG. 4A is formed by elementary units each comprising twotransverse rows, one of three heating elements and the following one oftwo pseudo heating elements.

The module shown diagrammatically in FIG. 4B comprises seven transverserows of elements: three transverse rows AA' of heating elements, twotransverse rows CC' of pseudo heating elements and two transverse rowsDD' comprising heating elements and pseudo heating elements. Byassociation of modules of this type according to a direction atright-angles to the transverse rows (that is to say parallel with thedirection of gas flow) and according to a direction parallel with thesetransverse rows, it is possible to form a longitudinal zone comprisinglayers FF' of heating elements, layers EE' of pseudo heating elementsand layers GG' comprising heating elements and pseudo heating elements.This module comprises substantially cylindrical heating elements and thepseudo heating elements situated on the periphery of the module (on thesides of the module) are solid elements which according to theirposition are substantially in the form of a half-cylinder orsubstantially in the form of a quarter of a cylinder, so that byjuxtaposition of the modules, pseudo elements are formed which aresubstantially cylindrical and which have a cross-section the area ofwhich is substantially equal to that of the cross-section of the heatingelements, or semi-cylindrical for those which are close to the walls ofthe longitudinal zone.

The module shown diagrammatically in FIG. 4C only differs from thatshown in 4B by the fact that it also comprises a refractory wall 22.

The association of modules of different types is entirely possible.Thus, for example, in the case of the choice of a geometry where theelements are in quincunx and form a longitudinal zone, it is possible toassociate modules such as those shown diagrammatically in FIGS. 3B and3C.

EXAMPLE 1

An indirect quenching horizontal reactor is used which has a totallength of 6.1 m and a rectangular cross-section of 1.4×2.89 m. The meansof heating this reactor consist of hairpin-form electric resistors ofKANTHAL make, of molybdenum bisilicide (MoSi₂) of the SUPERKANTHAL type;these resistors are enclosed by ceramic sheaths disposed concentricallyin relation to the centre of the circle embracing the resistors.

These sheaths are of silicon carbide produced by NORTON company. Theyare of the KRYSTON type and have an open porosity of 15% by volume.Closed at one end, each sheath encloses two hairpin resistors (FIG. 1B).These sheaths are disposed at right-angles to the direction ofcirculation of the charge (vertically) in parallel layers, and inperpendicular projection they form a cluster of square configuration.The length of each arm of the hairpin of the electrical resistor is 1.4m and its diameter is 9 mm. The ceramic sheaths are 1.4 m long, theiroutside diameter is 150 mm and their inside diameter is 130 mm; thedistance separating two adjacent sheaths is 10 mm.

The first part of the heating zone, 3.7 m long, comprises 18 layers ofresistors, each layer comprising 23 sheaths; in this zone, the chargewhich is preheated to 800° C., is raised to 1200° C. This zone isregulated thermally by means of thermocouples disposed in the spaces inwhich the charge circulates.

The second part of the heating zone, adjacent to the first part, is 2.4m long; it consists of 18 layers of 15 sheaths, disposed in the same wayas in the first part of the heating zone. This zone is constituted by 5heating sections which are independently regulated, which make itpossible to maintain the temperature in this zone at 1200° C. plus orminus 10° C.

The effluent gases are cooled in a first stage to 800° C. by indirectexchange with the gases of the charge; other temperature exchangers thenmake it possible to lower their temperature to approx. 300° C.

Used as a charge is methane diluted with hydrogen in a ratio of 1:1 byvolume. This mixture is preheated to 800° C. and cracked in theabove-described reactor. The absolute pressure of the gas mixture in thereactor is maintained substantially constant and equal to 0.125 MPa.Substantially pure hydrogen is introduced into the resistor space inorder to obtain and maintain in this space an absolute pressure which issubstantially constant and equal to 0.130 MPa.

After cooling to ambient temperature, for 200 moles of equivolumicmixture of methane and hydrogen, the following quantities of principalproducts are obtained:

    ______________________________________    PRODUCTS      QUANTITIES    ______________________________________    H.sub.2             143    moles    CH.sub.4            70     moles    C.sub.2 H.sub.2     6      moles    C.sub.2 H4          4      moles    benzene             0.75   moles    coke                54     grams    ______________________________________

EXAMPLE 2

An indirect quenching horizontal reactor is used which has a totallength of 4.31 m with a rectangular cross-section of 1.4×2.94 m. Themeans of heating consist of hairpin-shape electrical resistors ofKANTHAL make, of molybdenum bisilicide (MoSi₂) of the SUPERKANTHAL type;these resistors are enclosed in ceramic sheaths disposed concentricallyin relation to the centre of the circle embracing the resistors.

These sheaths are of silicon carbide produced by NORTON company. Theyare of the KRYSTON type and have an open porosity of 15% by volume.Closed at one end, each sheath encloses two hairpin resistors (FIG. 1B).These sheaths are disposed at right-angles to the direction ofcirculation of the charge (vertically) in parallel layers and inperpendicular projection they form a cluster of square configuration.The length of each arm of the hairpin of the electrical resistancemeasures 1.4 m and its diameter is 9 mm. The ceramic sheaths are 1.4 mlong. Their outside diameter is 150 mm and their inside diameter 130 mm;the distances Eg and Et (FIG. 1A) separating two adjacent sheaths are 10mm. The reactor comprises three longitudinal zones each comprising 6layers of heating elements separated by a wall of electrically fusedalumina-based refractory cement. The distance Ee (FIG. 1A) between thesheaths and the wall or size of the passages is 10 mm. The walls have,at their thinnest part, a thickness Ep (FIG. 1A) of 15 mm. Thus, thereactor comprises 18 layers of 27 heating elements and 2 walls.

The first part of the heating zone, 1.75 m long, comprises 18 layers ofresistors, each layer comprising 11 sheaths; in this zone, the chargepreheated to 1000° C. is raised to 1200° C. This zone is regulatedthermally by means of thermocouples disposed in the spaces in which thecharge circulates. The second part of the heating zone adjacent to thesaid first part is 2.56 m long; it is constituted by 18 layers of 16sheaths disposed in the same way as in the first part of the heatingzone. This zone is constituted by three heating sections which areindependently regulated, making it possible to maintain the temperaturein this zone at 1200° C. plus or minus 10° C.

The effluent gases are cooled in a first stage to 800° C. by indirectexchange with the gases of the charge; other temperature exchangers makeit possible then to lower their temperature to approx. 300° C.

Used as a charge is methane diluted with hydrogen in a volumetric ratioof 1:1. This mixture is preheated to 1000° C. and cracked in theabove-mentioned reactor. The absolute pressure of the gaseous mixture inthe reactor is maintained substantially constant and is equal to 0.125MPa. Substantially pure hydrogen is introduced into the resistor spacein order to obtain and maintain in this space a substantially constantabsolute pressure equal to 0.130MPa. After cooling to ambienttemperature, per 200 moles of equivolumic mixture of methane andhydrogen, the following quantities of principal products are obtained:

    ______________________________________    PRODUCTS      QUANTITIES    ______________________________________    H.sub.2             142.5  moles    CH.sub.4            70     moles    C.sub.2 H.sub.2     6.3    moles    C.sub.2 H.sub.4     4      moles    benzene             0.74   moles    coke                50     grams    ______________________________________

We claim:
 1. An apparatus comprising a reactor of elongated shape, comprising a square or rectangular cross-section and a longitudinal axis, said reactor having a first end, a second end, inlet means at said first end for supplying a gaseous mixture, outlet means at said second end for discharging effluent, and means for supplying a cooling medium,said reactor further comprising at least one longitudinal zone between two walls one of refractory material adjacent said first end, at least one of said walls being disposed between outside walls of said reactor wherein said two refractory walls are substantially parallel to each other and substantially parallel to said axis of said reactor, said at least one longitudinal zone comprising a plurality of elements disposed in at least two layers, said layers being substantially parallel to each other, and substantially parallel to said axis of said reactor, wherein said elements in at least one of said layers comprises a series of sheaths inside which are electric heating means, thereby forming a series of heating elements, said elements in at least two layers being positioned to define passages for circulation of gas between said elements, between said layers of elements, and/or between said elements and walls, whereby said electric heating means and said sheaths are capable of heating said passages in successive independent sections which are substantially perpendicular to said axis of said reactor, each of said sections comprising at least one transverse row of elements, said reactor further comprising means for automatic control and modulation of heating connected to said electric heating means, and said reactor further comprising, adjacent said second end, means for cooling effluent connected to said means for supplying cooling medium.
 2. An apparatus according to claim 1, wherein:each longitudinal zone is formed by juxtaposition of a series of constituent modules of square or rectangular cross-section, each module comprising at least two elements forming a transverse row, at least one of said elements being formed by a sheath inside which electric heating means is provided and constitutes a heating element, said elements being disposed to define passages for circulation of gas between said elements and/or between said elements and said walls of said zone, and said modules being juxtaposed, whereby said elements form at least two layers, substantially parallel to each other and substantially parallel with said axis of said reactor, said layers being positioned between two walls of refractory material which are substantially parallel other and substantially parallel with said axis of said reactor.
 3. An apparatus according to claim 2, wherein said constituent modules comprise at least two transverse rows of two or three heating elements disposed so that juxtaposition of said modules provides at least two layers of heating elements, said layers being perpendicular to said axis of said reactor and said elements forming in transverse projection a cluster of elements of triangular, square, or rectangular configuration.
 4. An apparatus according to claim 2, wherein said constituent modules comprise at least two transverse rows, of which at least one is formed by heating elements and of which at least one other, contiguous upon a row of said heating elements, is formed by pseudo heating elements of refractory material, said elements being disposed whereby juxtaposition of said modules provides at least two layers of elements, said layers being perpendicular to said axis of said reactor and said elements forming in transverse projection a cluster of elements of triangular, square, or rectangular configuration.
 5. An apparatus according to claim 4, wherein said constituent module comprises pseudo heating elements on its periphery, said pseudo heating elements having a cross-section whereby, as a result of juxtaposition of said modules, said pseudo heating elements situated at the edge of said modules form pseudo elements having substantially the same cross-section as said heating elements, said pseudo heating elements positioned on a single edge of said module having a cross-section equal to approximately half the cross-section of said heating elements and said pseudo heating elements positioned on two edges of said module having a cross-section equal to approximately one-quarter the cross-section of said heating elements.
 6. An apparatus according to claim 1, wherein said reactor comprises at least two of said longitudinal zones formed by juxtaposition in the longitudinal direction and, in the transverse direction, said longitudinal zones are each a series of modules, said modules comprising at least one longitudinal wall of refractory material.
 7. An apparatus according to claim 1, wherein each wall of refractory material separating two adjacent longitudinal zones comprises at least one means permitting balancing of pressures between said two adjacent longitudinal zones.
 8. An apparatus according to claim 1, wherein: said elements comprise heating elements and pseudo heating elements. 